1. Field of the Inventions
The invention relates to medical devices for treating epilepsy, and more particularly to a system incorporating an implantable device capable of causing changes in cerebral blood flow.
2. Description of the Related Art
Epilepsy, a neurological disorder characterized by the occurrence of seizures (specifically episodic impairment or loss of consciousness, abnormal motor phenomena, psychic or sensory disturbances, or the perturbation of the autonomic nervous system), is debilitating to a great number of people. It is believed that as many as two to four million Americans may suffer from various forms of epilepsy. Research has found that its prevalence may be even greater worldwide, particularly in less economically developed nations, suggesting that the worldwide figure for epilepsy sufferers may be in excess of one hundred million.
Because epilepsy is characterized by seizures, its sufferers are frequently limited in the kinds of activities they may participate in. Epilepsy can prevent people from driving, working, or otherwise participating in much of what society has to offer. Some epilepsy sufferers have serious seizures so frequently that they are effectively incapacitated.
Furthermore, epilepsy is often progressive and can be associated with degenerative disorders and conditions. Over time, epileptic seizures often become more frequent and more serious, and in particularly severe cases, are likely to lead to deterioration of other brain functions (including cognitive function) as well as physical impairments.
The current state of the art in treating neurological disorders, particularly epilepsy, typically involves drug therapy and surgery. The first approach is usually drug therapy.
A number of drugs are approved and available for treating epilepsy, such as sodium valproate, phenobarbital primidone, ethosuximide, gabapentin, phenytoin, and carbamazepine, as well as a number of others. Unfortunately, those drugs typically have serious side effects, especially toxicity, and it is extremely important in most cases to maintain a precise therapeutic serum level to avoid breakthrough seizures (if the dosage is too low) or toxic effects (if the dosage is too high). The need for patient discipline is high, especially when a patient's drug regimen causes unpleasant side effects the patient may wish to avoid.
Moreover, while many patients respond well to drug therapy alone, a significant number (at least 20-30%) do not. For those patients, surgery is presently the best-established and most viable alternative course of treatment.
Currently practiced surgical approaches include radical surgical resection such as hemispherectomy, corticectomy, lobectomy and partial lobectomy, and less-radical lesionectomy, transection, and stereotactic ablation. Besides being less than fully successful, these surgical approaches generally have a high risk of complications, and can often result in damage to eloquent (i.e., functionally important) brain regions and the consequent long-term impairment of various cognitive and other neurological functions. Furthermore, for a variety of reasons, such surgical treatments are contraindicated in a substantial number of patients. And unfortunately, even after radical brain surgery, many epilepsy patients are still not seizure-free.
Electrical stimulation is an emerging therapy for treating epilepsy. However, currently approved and available electrical stimulation devices do not perform any detection of neural activity and apply electrical stimulation to neural tissue surrounding or near implanted electrodes somewhat indiscriminately; they are not responsive to relevant neurological conditions. Responsive stimulation, in which neurological activity is detected and electrical stimulation treatment is applied selectively, is in clinical trials at the time of this writing.
The NeuroCybernetic Prosthesis (NCP) from Cyberonics, for example, applies continuous electrical stimulation to the patient's vagus nerve. This approach has been found to reduce seizures by about 50% in about 50% of patients. Unfortunately, a much greater reduction in the incidence of seizures is needed to provide substantial clinical benefit.
The Activa device from Medtronic is a pectorally implanted continuous deep brain stimulator intended primarily to treat Parkinson's disease. In operation, it continuously supplies an intermittent electrical pulse stream to a selected deep brain structure where an electrode has been implanted. Continuous stimulation of deep brain structures for the treatment of epilepsy has not met with consistent success. To be effective in terminating seizures, it is believed that one effective site where stimulation should be performed is near the focus of the epileptogenic region. The focus is often in the neocortex, where continuous stimulation above a certain level may cause significant neurological deficit with clinical symptoms including loss of speech, sensory disorders, or involuntary motion. Accordingly, and to improve therapeutic efficacy over indiscriminate continuous stimulation, research has been directed toward automatic responsive epilepsy treatment based on a detection of imminent seizure.
A typical epilepsy patient experiences episodic attacks or seizures. Those events, neurological states, and epileptiform activity evident on the EEG shall be referred to herein as “ictal”.
Most prior work on the detection and responsive treatment of seizures via electrical stimulation has focused on analysis of electroencephalogram (EEG) and electrocorticogram (ECoG) waveforms. In common usage, the term “EEG” is used to refer to signals representing aggregate neuronal activity potentials detectable via electrodes applied to a patient's scalp, though the term can also refer to signals obtained from deep in the patient's brain via depth electrodes and the like. Specifically, “ECoGs” refer to signals obtained from internal electrodes near the cortex of the brain (generally on or under the dura mater), but is often used to refer to direct brain signals from deeper structures as well; an ECoG is a particular type of EEG. Unless the context clearly and expressly indicates otherwise, the term “EEG” shall be used generically herein to refer to both EEG and ECoG signals, regardless of where in the patient's brain the electrodes are located.
It is generally preferable to be able to detect and treat a seizure at or near its beginning, or even before it begins. The beginning of a seizure is referred to herein as an “onset.” However, it is important to note that there are two general varieties of seizure onsets. A “clinical onset” represents the beginning of a seizure as manifested through observable clinical symptoms, such as involuntary muscle movements or neurophysiological effects such as lack of responsiveness. An “electrographic onset” refers to the beginning, of detectable electrographic activity indicative of a seizure. An electrographic onset will frequently occur before the corresponding clinical onset, enabling intervention before the patient suffers symptoms, but that is not always the case. In addition, there are changes in the EEG that occur seconds or even minutes before the electrographic onset that can be identified, and may be used to facilitate intervention before clear electrographic or clinical onsets occur. This capability would be considered seizure anticipation, in contrast to the detection of a seizure or its onset. Seizure anticipation is much like weather prediction—there is an indication the likelihood has increased that a seizure will occur, but when exactly it will occur, or even if it will occur is not certain.
U.S. Pat. No. 6,018,682 to Rise describes an implantable seizure warning system that implements a form of the Gotman system. See, e.g., J. Gotman, Automatic seizure detection: improvements and evaluation, Electroencephalogr. Clin. Neurophysiol. 1990; 76(4): 317-24. However, the system described therein uses only a single detection modality, namely a count of sharp spike and wave patterns within a time period, and is intended to provide a warning of impending seizure activity in spite of a lack of evidence that spike and wave activity is sufficiently anticipatory of seizures. This is accomplished with relatively complex processing, including averaging over time and quantifying sharpness by way of a second derivative of the signal. The Rise patent does not disclose how the signals are processed at a low level, nor does it explain detection criteria in any specific level of detail.
U.S. Pat. No. 6,016,449 to Fischell, et al. (which is hereby incorporated by reference as though set forth in full herein), describes an implantable seizure detection and treatment system. In the Fischell system, various detection methods are possible, all of which essentially rely upon the analysis (either in the time domain or the frequency domain) of processed EEG signals. Fischell's controller is preferably implanted intracranially, but other approaches are also possible, including the use of an external controller. The processing, and detection techniques applied in Fischell are generally well suited for implantable use. When a seizure is detected, the Fischell system applies responsive electrical stimulation to terminate the seizure, a capability that will be discussed in further detail below.
All of these approaches provide useful information, and in some cases may provide sufficient information for accurate detection and or anticipation of most imminent epileptic seizures.
It has been found that many clinical neurological disorders are associated with abnormal blood flow patterns in the brain. These include epilepsy, movement disorders, and psychiatric disorders. It would therefore be advantageous to employ a system or method to monitor such abnormal blood flow patterns, either in isolation or in connection with abnormal electrographic activity, to identify the status of the disease state and to monitor the short-term and/or long-term progression of the disease state with the intention of correcting the abnormal blood flow patterns to provide clinical benefit. Such monitoring is preferably accomplished within the therapy delivery device (often a neurostimulator) to automatically adjust therapy delivery to the patient to more optimally bring about beneficial changes in brain blood flow patterns either acutely or more long term. Therapy delivery may be direct brain electrical stimulation, spinal cord stimulation, brain stem or peripheral nerve stimulation, or may be magnetic stimulation, somatosensory stimulation, or drug delivery. However, monitoring may include means not included in the therapy delivery device, with therapy being adjusted by a clinician. Monitoring of the brain blood flow can be accomplished by the periodic application of non-invasive imaging techniques including SPECT, PET, SISCOM, infrared, ultrasound, or impedance techniques.
As is well known, it has been suggested that it is possible to treat and terminate seizures by applying electrical stimulation to the brain. See, e.g., U.S. Pat. No. 6,016,449 to Fischell et al., and H. R. Wagner, et al., Suppression of cortical epileptiform activity by generalized and localized ECoG desynchronization, Electroencephalogr. Clin. Neurophysiol. 1975; 39(5): 499-506. It has further been found that electrical stimulation can modulate blood flow in the brain. Cortical stimulation increases blood flow within hundreds of milliseconds at the site of stimulation (T. Matsuura et al., “Hemodynamics evoked by microelectrical direct stimulation in rat somatosensory cortex.” Comp. Biochem. Physiol. A. Mol. Integr. Physiol. September, 1999, 124(1): 47-52; see also S. Bahar et al., “THE RELATIONSHIP BETWEEN CEREBRAL BLOOD VOLUME AND OXYGENATION FOLLOWING BIPOLAR STIMULATION OF THE HUMAN CORTEX: EVIDENCE FOR AN INITIAL DIP,” AES 12-2004 New Orleans Poster Session). Stimulation of other brain structures or through the use of transcranial magnetic stimulation can produce patterns of blood flow changes including increases or reductions of blood flow) in targeted areas.
At the current time, there is no known implantable device that is capable of treating abnormal neurological conditions, including seizures, by changing cerebral perfusion either acutely or chronically. Furthermore, there is no known implantable device that is capable of detecting and or anticipating seizures or other neurological events based on cerebral perfusion conditions and changes therein, alone or in combination with other observed conditions. As anticipated herein, modulation of blood perfusion in the brain may be employed for acute or chronic treatment of neurological conditions.